Trying to identify what exactly a zombie is can be extremely tricky: so many different authors/game designers/artists have represented zombies in so many different ways that even within a media you can see significant differences. In my very limited exposure to zombies, I would define a zombie as a mindless, soulless creature that walks/stumbles around. They are vaguely human in shape, but I see them having a green or a blue-gray shade of skin. They rarely, if ever, eat and can survive without several or even all of their limbs. In order to kill them, you need to cut off their head.
I am sure that over this course I will expand this definition and may even entirely change it … only time will tell!